PROJECT SUMMARY Maternal obesity is a major public health crisis, as >50% of women enter pregnancy overweight or obese. This is alarming, as children of obese women are more likely to develop risk factors for metabolic disease (including obesity and insulin resistance) and reproductive disorders (such as earlier menarche in daughters and poor sperm quality in sons). Important early-life predictors of metabolic disease include birth weight and ponderal index (measured from birth weight and length), while early-life predictors of reproductive function include the ratio of the 2nd-to-4th finger lengths (2:4D) and the distance between the anus and genitals (anogenital distance, AGD). In humans, altered 2:4D is associated with various reproductive abnormalities, while early-life AGD, specifically, correlates with long-term reproductive health and capacity. While standard measures of fetal development, including birth weight and ponderal index, are altered in newborns of obese women, it is not known whether maternal obesity affects neonatal AGD or 2:4D. Therefore, we will leverage an ongoing pregnancy cohort that has already measured AGD and 2:4D, and that has various measures of maternal body composition, to investigate whether maternal obesity is associated with these early-life markers of perturbed reproductive development. Based on previous studies, we hypothesize that male infants of overweight/obese women will be feminized (have shorter AGD and longer 2:4D), while female infants of obese women will be masculinized (have longer AGD and shorter 2:4D). Altered fetal growth, as measured by birth weight or ponderal index, is a risk factors for later-life obesity, while newborn AGD and 2:4D are risk factors for altered reproductive capacity. Maternal levels of estrogen and testosterone, two important steroid hormones in pregnancy, are critical for fetal development and growth. For example, estrogen controls placental angiogenesis, which regulates nutrient transfer to the fetus, while elevated maternal testosterone in a sheep model of pregnancy was shown to cause intrauterine growth restriction. AGD and 2:4D, which are associated with reproductive capacity, are also sex steroid hormone-mediated; in males and females, both are markers of in utero testosterone or estrogen exposures, and are sensitive to hormonal disruption by maternal environmental factors. Findings from our pregnancy cohort suggest that maternal obesity is also an endocrine disruptor, as estrogen and testosterone were lower in overweight/obese pregnant women compared to normal-weight women, and associations with testosterone were only seen in women carrying males. It is not known whether these hormonal shifts in obese women are partially responsible for the well-known associations of maternal obesity with fetal developmental outcomes (birth weight and ponderal index), or for the proposed associations with 2:4D ratio and AGD. Therefore, we will investigate this important biological mechanism in the current study.